Optical disk apparatus and optical disk identifying method

ABSTRACT

In an optical disc drive capable of recording and playing back different kinds of optical discs, an actuator driving means brings an objective lens close to the optical disc, focus error signals are generated in front of and behind a position where leaser beam comes into focus on a signal surface of the optical disc, optical disc discriminating means discriminates a thickness of the optical disc from a difference in waveform of the focus error signal which is generated by focus deviation between leaser beam which passes through an outer periphery of the objective lens and leaser beam which passes through an inner periphery of the objective lens, and discriminates a kind of the optical disc from the thickness of the optical disc.

TECHNICAL FIELD

The present invention relates to an optical disc drive capable ofrecording and playing back different kinds of optical discs, and adiscriminating method of optical disc for discriminating kinds ofoptical discs.

BACKGROUND TECHNIQUE

There is a conventionally known discriminating method of kinds ofoptical discs based a difference in base material thicknesses of opticaldiscs.

FIG. 7 shows such a conventional discriminating method of optical discs.

First, a discriminating method of optical discs disclosed in JapanesePatent Application Laid-open No. H8-287588 will be explained.

In this conventional technique, if an objective lens 1 shown in FIG. 7is brought close to a CD 52 or a DVD 53, a sum signal 55 having a smallamplitude based on light reflected from the disc is obtained at aposition where laser beam comes into focus on a base material surface54. Then, if the objective lens 1 is brought further close to the CD 52or the DVD 53, another sum signal 57 having a large amplitude based onlight reflected from the disc is obtained at a position where laser beamcomes into focus on a signal surface 56. These sum signals 55 and 57 arecompared with level 1, and differential time T1 between the instant whenthe base material surface 54 was detected and the instant when thesignal surface 56 is detected is measured based on a binary type sumsignal. The base material thickness of the optical disc is recognizeddepending upon the length of the measured differential time T1, and akind of the optical disc is discriminated based on the recognized basematerial thickness.

Next, a discriminating method of optical discs disclosed in JapanesePatent Application Laid-open No. H11-149640 will be explained.

In this conventional technique, the following method is proposed so asto enhance the discriminating precision of discs.

That is, an S-shaped signal portion 58 in a focus error signal generatedbased on light reflected from the optical disc is compared with leve 12and leve 13, there by generating an S-shaped positive detection signaland an S-shaped negative detection signal, and detection differentialtime T2 between an S-shaped positive signal and an S-shaped negativesignal is measured. The differential time T1 between the instant whenthe base material surface 54 was detected and the instant when thesignal surface 56 is measured. Then, T1/T2 is calculated, therebynormalizing differential time between the instant when the base materialsurface 54 was detected and the instant when the signal surface 56 isdetected. This conventional technique uses the normalized T1/T2 value,thereby eliminating influence of sensitive variation in a focusdirection of an actuator.

When a surface of the optical disc is deflected, however, since the basematerial surface and the signal surface are vertically deflected, sumsignals 61 based on light reflected from the base material surface andsum signals 62 based on light reflected from the signal surface aregenerated at a plurality of locations, and positions are varied in thelongitudinal direction. For this reason, this conventional technique hasa problem that the time can not be measured precisely and discriminationerror of discs is generated. Further, there are problems that the sumsignal 61 of the base material surface has a small amplitude, noise iserroneously detected as the sum signal 61 of the base material surface,time can not be measured precisely and discrimination error of discs isgenerated.

According to a discriminating method of optical discs disclosed inJapanese Patent Application Laid-open No. H11-149640, two signals, i.e.,a focus error signal and a sum signal are required, time measuring meansof T1, binary means of the focus error signal and binary means of thesum signal are required, and the structure becomes complicated.

Thereupon, it is an object of the present invention to preciselydiscriminate kinds of discs even when a focus position is varied bydeflection of surface and a plurality of S-shapes of the focus errorsignal are generated.

It is another object of the invention to eliminate influence ofvariation in laser power and reflection ratio of optical discs, and todiscriminate discs precisely.

DISCLOSURE OF THE INVENTION

A first aspect of the present invention provides an optical disc drivecapable of recording and playing back different kinds of optical discs,comprising a pickup for irradiating the optical disc with leaser beam,laser control means for controlling the irradiation of the leaser beam,actuator driving means for moving an objective lens which constitutesthe pickup in a focus direction, focus detection photoreceiving meansfor detecting a focus state by means of light reflected from the opticaldisc, FE signal measuring means for measuring an amplitude of a focuserror signal which is generated by the focus detection photoreceivingmeans, and optical disc discriminating means for discriminating theoptical disc from the amplitude measured by the FE signal measuringmeans, wherein the actuator driving means brings the objective lensclose to the optical disc, focus error signals are generated in front ofand behind a position where the leaser beam comes into focus on a signalsurface of the optical disc, the optical disc discriminating meansdiscriminates a thickness of the optical disc from a difference inwaveform of the focus error signal which is generated by focus deviationbetween leaser beam which passes through an outer periphery of theobjective lens and leaser beam which passes through an inner peripheryof the objective lens, and discriminates a kind of the optical disc fromthe thickness of the optical disc.

According to the first aspect, the thickness of the optical disc isdiscriminated from the difference in waveform of the focus error signalwhich is generated by focus deviation between leaser beam which passesthrough an outer periphery of the objective lens and leaser beam whichpasses through an inner periphery of the objective lens. Therefore, evenif the focus position is varied due to the surface deflection and aplurality of S-shapes of the focus error signals are generated, thewaveform of the focus error signal is not varied only if the thicknessof the optical disc is the same. Further, if the thicknesses of theoptical discs are different, the waveform of the focus error signal isdifferent and thus, it is possible to discriminate a kind of the discprecisely. Since it is unnecessary to measure the time, the timecounting means is not required. It is also unnecessary to detect a sumsignal having small amplitude of the base material surface, a case inwhich noise is erroneously detected and an optical disc is erroneouslydiscriminated is not caused, and it is possible to preciselydiscriminate the optical disc.

According to a second aspect of the present invention, in the opticaldisc of the first aspect, when a maximum value of the amplitude of thefocus error signal is defined as FEmax and a minimum value of theamplitude of the focus error signal is defined as FEmin, an absolutevalue of the FEmax and an absolute value of the FEmin are compared witheach other, thereby discriminating the thickness of the optical disc.

With this aspect, even if the focus position is varied due to thesurface deflection and the plurality of S-shapes of the focus errorsignals are generated, since the magnitude of each of the maximum valueand minimum value of the amplitude is not varied, it is possible todiscriminate a kind of a disc precisely by comparing the absolute valueof FEmax and absolute value of FEmin with each other.

According to a third aspect of the present invention, in the opticaldisc of the first aspect, when a maximum value of the amplitude of thefocus error signal is defined as FEmax and a minimum value of theamplitude of the focus error signal is defined as FEmin, the thicknessof the optical disc is discriminated by means of a value(FEmax+FEmin)/(FEmax−FEmin).

With this aspect, a ratio between a sum and a difference of the maximumvalue FEmax and the minimum value FEmin of the focus error signal isobtained and normalized, thereby eliminating the influence of thevariation of laser power and the variation of the reflection ratio ofthe optical disc, and it is possible to precisely discriminate the disc.

According to a fourth aspect of the present invention, in the opticaldisc of the first aspect, when a maximum value of the amplitude of thefocus error signal is defined as FEmax and a minimum value of theamplitude of the focus error signal is defined as FEmin, a differencebetween an absolute value of the FEmax and an absolute value of theFEmin is obtained, and this difference is compared with a predetermineddiscrimination value, thereby discriminating the thickness of theoptical disc.

With this aspect, even if the focus position is varied due to thesurface deflection and the plurality of S-shapes of the focus errorsignals are generated, since the magnitude of each of the maximum valueand minimum value of the amplitude is not varied, it is possible todiscriminate a kind of a disc precisely by comparing the differencebetween the absolute value of FEmax and absolute value of FEmin with thepredetermined discrimination value.

A fifth aspect of the present invention provides an optical disc drivecapable of recording and playing back different kinds of optical discs,comprising a pickup for irradiating the optical disc with leaser beam,laser control means for controlling the irradiation of the leaser beam,actuator driving means for moving an objective lens which constitutesthe pickup in a focus direction, focus detection photoreceiving meansfor detecting a focus state by means of light reflected from the opticaldisc, FE signal measuring means for measuring an amplitude of a focuserror signal which is generated by the focus detection photoreceivingmeans, and optical disc discriminating means for discriminating theoptical disc from the amplitude measured by the FE signal measuringmeans, wherein the actuator driving means brings the objective lensclose to the optical disc, focus error signals are generated in front ofand behind a position where the leaser beam comes into focus on a signalsurface of the optical disc, the optical disc discriminating meansdiscriminates a thickness of the optical disc from a symmetry between apositive waveform and a negative waveform of an amplitude of the focuserror signal which is generated by focus deviation between leaser beamwhich passes through an outer periphery of the objective lens and leaserbeam which passes through an inner periphery of the objective lens, anddiscriminates a kind of the optical disc from the thickness of theoptical disc.

According to the fifth aspect, the thickness of the optical disc isdiscriminated from a symmetry between a positive waveform and a negativewaveform of an amplitude of the focus error signal which is generated byfocus deviation between leaser beam which passes through an outerperiphery of the objective lens and leaser beam which passes through aninner periphery of the objective lens. Therefore, even if the focusposition is varied due to the surface deflection and a plurality ofS-shapes of the focus error signals are generated, the waveform of thefocus error signal is not varied only if the thickness of the opticaldisc is the same. Further, if the thicknesses of the optical discs aredifferent, the waveform of the focus error signal is different and thus,it is possible to discriminate a kind of the disc precisely. Since it isunnecessary to measure the time, the time counting means is notrequired. It is also unnecessary to detect a sum signal having smallamplitude of the base material surface, a case in which noise iserroneously detected and an optical disc is erroneously discriminated isnot caused, and it is possible to precisely discriminate the opticaldisc.

A sixth aspect of the present invention provides an optical discdiscriminating method for discriminating a kind of an optical disc,comprising a driving step for bringing an objective lens close to theoptical disc while irradiating the optical disc with leaser beam, an FEsignal measuring step for measuring an amplitude of a focus error signalduring the driving step and for storing a maximum value FEmax and aminimum value FEmin of the focus error signal, and a discriminating stepfor comparing a difference between an absolute value of the maximumvalue FEmax and an absolute value of the minimum value FEmin which arestored in the FE signal measuring step with a previously storeddiscrimination value, wherein the kind of the optical disc isdiscriminated in the discriminating step.

With this aspect, even if the focus position is varied due to thesurface deflection and a plurality of S-shapes of the focus errorsignals are generated, since the magnitude of each of the maximum valueand minimum value of the amplitude is not varied, it is possible todiscriminate a kind of a disc precisely. Since it is unnecessary tomeasure the time, the time counting means is not required. It is alsounnecessary to detect a sum signal having small amplitude of the basematerial surface, a case in which noise is erroneously detected and anoptical disc is erroneously discriminated is not caused, and it ispossible to precisely discriminate the optical disc.

According to a seventh aspect of the present invention, in the opticaldisc discriminating method of the sixth aspect, in the discriminatingstep, (FEmax+FEmin)/(FEmax−FEmin) is calculated, the calculated(FEmax+FEmin)/(FEmax−FEmin) is compared with the previously storeddiscrimination value, thereby discriminating the kind of the opticaldisc.

With this aspect, a ratio between a sum and a difference of the maximumvalue FEmax and the minimum value FEmin of the focus error signal isobtained and normalized, thereby eliminating the influence of thevariation of laser power and the variation of the reflection ratio ofthe optical disc, and it is possible to precisely discriminate the disc.

According to an eighth aspect of the present invention, in the opticaldisc discriminating method of the sixth or seventh aspect, a valueobtained by calculating the maximum value FEmax and the minimum valueFEmin based on the focus error signal at a position where the leaserbeam comes into focus is used as the discrimination value.

With this aspect, since the focus error signal at the position where theleaser beam comes into focus is used as a reference, it is possible todiscriminate a kind of the disc more precisely.

According to a ninth aspect of the present invention, in the opticaldisc drive of any one of the first to fifth aspects, the optical discdrive uses a pickup in which a focus deviation is generated betweenleaser beam which passes through an outer periphery of the objectivelens and leaser beam which passes through an inner periphery of theobjective lens by a difference in base material thickness of opticaldisc.

With this aspect, since the optical disc drive uses a pickup in which afocus deviation is generated between leaser beam which passes through anouter periphery of the objective lens and leaser beam which passesthrough an inner periphery of the objective lens by a difference in basematerial thickness of optical disc, the amplitudes of the maximum valueFEmax and the minimum value FEmin can be made different from each other,and it is possible to discriminate a kind of the disc more precisely.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical disc drive according to anembodiment of the present invention.

FIG. 2 is a diagram for explaining a generating method of an FE signalaccording to the embodiment.

FIG. 3 is a diagram for explaining influence of a difference in basematerial thickness of an optical disc exerted on the FE signal.

FIG. 4 is a diagram for explaining a normalizing processing in adiscriminating method of optical discs according to the embodiment.

FIG. 5 is a flowchart of the discriminating method of optical discsaccording to the embodiment.

FIG. 6 is a flowchart of the discriminating method of optical discsaccording to another embodiment.

FIG. 7 is a diagram for explaining a conventional discriminating methodof optical discs.

FIG. 8 is a diagram for explaining a signal state when a surface of anoptical disc is deflected.

BEST MODE FOR CARRYING OUT THE INVENTION

An optical disc drive according to an embodiment of the presentinvention will be explained using FIGS. 1 to 3.

FIG. 1 is a block diagram of the optical disc drive according to theembodiment. A pickup 10 can obtain a playback signal from differentkinds of optical discs such as CDs and DVDs and write a recording signalinto different kinds of optical discs such as CDs and DVDs. The pickup10 includes a semiconductor laser 5, a condensing lens 2, an objectivelens 1, a polarizing hologram 3, a tracking detection photoreceivingmeans 4 and a focus detection photoreceiving means 6. The semiconductorlaser 5 emits laser having appropriate power under control of lasercontrol means 7. In order to resolve a focus error and a tracking error,the objective lens 1 can respectively move in a focus direction and adiameter direction of the disc in response to a driving signal from anactuator driving means 11.

The condensing lens 2 converts laser beam irradiated from thesemiconductor laser 5 into parallel light. When DVD laser beam isconverted into parallel light, the CD laser beam is slightly diffused.Light which passed through the condensing lens 2 forms a light spot onthe optical disc 8 by the objective lens 1. On the other hand, lightreflected from the optical disc 8 passes through the objective lens 1and the condensing lens 2, and is diffracted into focus detectingreflection light and tracking detecting reflection light by means of thepolarizing hologram 3, and the focus detecting reflection light collidesagainst the focus detection photoreceiving means 6. The FE signalmeasuring means 12 measures a maximum value FEmax 15 and a minimum valueFEmin 16 of the focus error signal which was outputted from the focusdetection photoreceiving means 6 as a voltage value or a current value.The optical disc discriminating means 13 can discriminate a kind of theoptical disc from a difference in waveform of the S-shaped signal basedon the focus error signal at a position where the laser beam comes intofocus. Here, the optical disc discriminating means 13 discriminates thekind of the optical disc based on the maximum value FEmax and theminimum value FEmin of the amplitude of the focus error signal measuredby the FE signal measuring means 12. Here, the focus error signal at aposition where the laser beam comes into focus is defined as 0(reference), and values of the maximum value FEmax and the minimum valueFEmin are calculated. Control means 14 controls the actuator drivingmeans 11, the optical disc discriminating means 13 and the laser controlmeans 7.

Next, one method for generating the FE signal will be explained withreference to FIG. 2.

The focus detecting reflection light diffracted by the condensing lens 2is divided into four, and each divided light is further divided intolight brought into focus in front of the focus detection photoreceivingmeans 6 and light brought into focus behind the focus detectionphotoreceiving means 6, and each light collides against the focusdetection photoreceiving means 6. Regions A and B are formed on asurface of the focus detection photoreceiving means 6 by a parting line.Voltage with respect to light quantity colliding against the region B bya photoelectric effect is outputted as FE1, and voltage with respect tolight quantity colliding against the region A by a photoelectric effectis outputted as FE2. The focus error signal is generated by a differencebetween the two voltages FE1-FE2.

Next, influence of a difference in base material thickness of an opticaldisc exerted on the FE signal will be explained with reference to FIG.3.

Deviation is generated in laser beam condensed by the objective lens 1at a focus position where the laser beam passes through an outerperiphery of the objective lens 1 located far from an optical axis andis condensed and a focus position where the laser beam passes through aninner periphery of the objective lens 1 located in the vicinity of theoptical axis and is condensed. This is called spherical aberration.

Unlike a wave front 24 in the air when light is converged through a DVD23 having a base material thickness of 0.6 mm without sphericalaberration, in the case of a wave front 25 in the air when light isconverged through a CD 20 having a base material thickness of 1.22 mmwithout spherical aberration, the focus position is delayed on thatportion of the wave front 25 which is closer to its outer periphery andwhich is further from the optical axis.

In order to make it possible to playback and record two or more kinds ofoptical discs, the pickup 10 of the optical disc drive according to thisembodiment can irradiate two or more kinds of laser beams havingdifferent wavelengths. The pickup 10 may be a single pickup capable ofhandling the two or more kinds of optical discs by switching an opticalsystem or a signal outputting system, but the pickup 10 may comprise aplurality of pickups. Here, first laser beam irradiated from the pickup10 is called DVD playback laser beam, and second leaser beam irradiatedfrom the pickup 10 is called CD playback leaser beam.

The first laser beam for DVD playback is designed such that thespherical aberration becomes minimum with respect to the DVD 23, and thesecond laser beam for CD playback is designed such that the sphericalaberration becomes minimum with respect to the CD 20.

When using an objective lens 1 which is designed such that the sphericalaberration of the first laser beam becomes minimum in the focus positionof the DVD 23, the light is slightly diverged by the condensing lens 2such that the focus position of leaser beam passing through the outerperiphery is delayed with respect to the second laser beam which playsback the CD 20. Here, since the wave front at the outer periphery isdelayed if the DVD 23 is irradiated with second laser beam, lightpassing through the outer periphery comes into focus in front of thefocus position of light passing through the inner periphery.

Here, when the objective lens 1 is brought close to the CD 20 whileirradiating second laser beam, no delay is generated in leaser beampassing through the inner periphery and leaser beam passing through theouter periphery. Therefore, FEmax 29 and FEmin 30 of an FE signal 28formed of light which passed through the entire surface of the objectivelens 1 become amplitude signals having substantially the same size. Onthe other hand, when the objective lens 1 is brought closer to the DVD23, leaser beam passing through the outer periphery is delayed fromleaser beam passing through the inner periphery. Therefore, FE signal 33formed of light which passed through the entire surface of the objectivelens 1 becomes a signal in which the value FEmax 34 is small and thevalue FEmin 35 is large. Therefore, if absolute values |FEmax| and|FEmin| are compared with each other, it is possible to discriminate thethickness of the disc.

Next, normalizing processing in the discriminating method of opticaldiscs according to the embodiment of the invention will be explainedusing FIG. 4.

The FEmax and FEmin are largely varied by variation in laser power andvariation in reflection ratio of optical discs. To eliminate theinfluence of the variations, the following normalizing processing iscarried out in the optical disc discriminating means 13.

An intermediate level (FEmax+FEmin)/2 of a focus error signal is dividedby an amplitude value (FEmax−FEmin) of the focus error signal, therebyobtaining a vertical balance value (FEmax+FEmin)/(FEmax−FEmin) of thefocus error signal. Since the vertical balance value cancels theamplitude level of the focus error signal, it is possible to reduce theinfluence of the variation in laser power and variation of thereflection ratio of the optical discs. The vertical balance value(FEmax+FEmin)/(FEmax−FEmin) of the focus error signal is compared with apreset discrimination value, thereby discriminating the optical disc.

Next, the optical disc discriminating method of the embodiment will beexplained using FIG. 5. The optical disc discriminating method of thisembodiment can be carried out in the optical disc drive shown in FIG. 1.

FIG. 5 is a flowchart of the optical disc discriminating method of thisembodiment.

The optical disc 8 is irradiated with leaser beam (S1) and in thisstate, the objective lens 1 existing in the pickup 10 is driven in afocus direction (S2). While the objective lens 1 is driven, the FEsignal is measured (S3), the FE signal maximum value is stored in theFEmax (S4), the FE signal minimum value is stored in the FEmin (S5).Then, a difference between an absolute value of the FEmax and anabsolute value of the FEmin are compared with a predetermineddiscrimination value (S6), and if the difference is greater than thepredetermined discrimination value, the disc is discriminated as being aCD (S7), and if the difference is smaller than the discrimination value,the disc is discriminated as being a DVD (S8).

Next, an optical disc discriminating method according to anotherembodiment of the present invention will be explained using FIG. 6.

FIG. 6 is a flowchart of the optical disc discriminating method of thisembodiment.

The optical disc 8 is irradiated with leaser beam (SI) and in thisstate, the objective lens 1 existing in the pickup 10 is driven in afocus direction (S2). While the objective lens 1 is driven, the FEsignal is measured (S3), the FE signal maximum value is stored in theFEmax (S4), the FE signal minimum value is stored in the FEmin (S5).Then, (FEmax+FEmin)/(FEmax−FEmin) is calculated (S9), and a result ofthe calculation is compared with a preset discrimination value (S10). Ifthe result of the calculation is greater than the predetermineddiscrimination value, the disc is discriminated as being a CD (S7), andif the difference is smaller than the discrimination value, the disc isdiscriminated as being a DVD (S8).

As the pickup, since a pickup in which a deviation between focus ofleaser beam passing through the outer periphery and focus of leaser beampassing through the inner periphery is generated due to a difference inbase material thicknesses of optical discs is used, amplitudes of themaximum value FEmax and the minimum value FEmin can be made differentfrom each other, and it is possible to discriminate the kinds of discsmore precisely.

Although two kinds of discs having the base material thickness of 0.6 mmand the base material thickness of 1.2 mm are discriminated in the aboveembodiment, another base material thickness may be employed, and kindsof three or more discs having different base material thicknesses canalso be discriminated, and the invention can also be applied to a basematerial thickness A, a base material thickness B and a base materialthickness C (in a relation of A>B>C).

The kinds of optical discs are discriminated based on the maximum valueFEmax and the minimum value FEmin of the amplitude of the focus errorsignal in the above embodiment. Alternatively, it is also possible toemploy the following configuration. That is, symmetry between a positivewaveform and a negative waveform of an amplitude of a focus error signalis obtained as a difference in waveforms of a S-shaped signal, then, adegree of the symmetry between the positive waveform and the negativewaveform may be compared with a preset reference value. Further, asanother means for discriminating a difference in waveforms of theS-shaped signal, an amplitude ration of the focus error signal isobtained, then, this amplitude ration maybe compared with a presetreference value. Further, as another means for discriminating adifference in waveforms of the S-shaped signal, a ratio of anintermediate value between a maximum value and a minimum value of thefocus error signal and an amplitude of the focus error signal isobtained, then, this ratio may be compared with a preset referencevalue.

Industrial Applicability

According to the present invention, as described above, it is possibleto discriminate kinds of discs precisely without being affected bysurface deflection, noise, variation in laser power output and the like,and without the need of time-counting means.

1. An optical disc drive capable of recording and playing back differentkinds of optical discs, comprising a pickup for irradiating the opticaldisc with leaser beam, laser control means for controlling theirradiation of the leaser beam, actuator driving means for moving anobjective lens which constitutes said pickup in a focus direction, focusdetection photoreceiving means for detecting a focus state by means oflight reflected from said optical disc, FE signal measuring means formeasuring an amplitude of a focus error signal which is generated bysaid focus detection photoreceiving means, and optical discdiscriminating means for discriminating said optical disc from theamplitude measured by said FE signal measuring means, wherein saidactuator driving means brings said objective lens close to the opticaldisc, focus error signals are generated in front of and behind aposition where the leaser beam comes into focus on a signal surface ofthe optical disc, said optical disc discriminating means discriminates athickness of the optical disc from a difference in waveform of the focuserror signal which is generated by focus deviation between leaser beamwhich passes through an outer periphery of said objective lens andleaser beam which passes through an inner periphery of said objectivelens, and discriminates a kind of said optical disc from the thicknessof said optical disc.
 2. The optical disc drive according to claim 1,wherein when a maximum value of the amplitude of said focus error signalis defined as FEmax and a minimum value of the amplitude of said focuserror signal is defined as FEmin, an absolute value of the FEmax and anabsolute value of the FEmin are compared with each other, therebydiscriminating the thickness of the optical disc.
 3. The optical discdrive according to claim 1, wherein when a maximum value of theamplitude of said focus error signal is defined as FEmax and a minimumvalue of the amplitude of said focus error signal is defined as FEmin,the thickness of said optical disc is discriminated by means of a value(FEmax+FEmin)/(FEmax−FEmin).
 4. The optical disc drive according toclaim 1, wherein when a maximum value of the amplitude of said focuserror signal is defined as FEmax and a minimum value of the amplitude ofsaid focus error signal is defined as FEmin, a difference between anabsolute value of the FEmax and an absolute value of the FEmin isobtained, and this difference is compared with a predetermineddiscrimination value, thereby discriminating the thickness of saidoptical disc.
 5. An optical disc drive capable of recording and playingback different kinds of optical discs, comprising a pickup forirradiating the optical disc with leaser beam, laser control means forcontrolling the irradiation of the leaser beam, actuator driving meansfor moving an objective lens which constitutes said pickup in a focusdirection, focus detection photoreceiving means for detecting a focusstate by means of light reflected from said optical disc, FE signalmeasuring means for measuring an amplitude of a focus error signal whichis generated by said focus detection photoreceiving means, and opticaldisc discriminating means for discriminating said optical disc from theamplitude measured by said FE signal measuring means, wherein saidactuator driving means brings said objective lens close to the opticaldisc, focus error signals are generated in front of and behind aposition where the leaser beam comes into focus on a signal surface ofthe optical disc, said optical disc discriminating means discriminates athickness of the optical disc from a symmetry between a positivewaveform and a negative waveform of an amplitude of the focus errorsignal which is generated by focus deviation between leaser beam whichpasses through an outer periphery of said objective lens and leaser beamwhich passes through an inner periphery of said objective lens, anddiscriminates a kind of said optical disc from the thickness of saidoptical disc.
 6. An optical disc discriminating method fordiscriminating a kind of an optical disc, comprising a driving step forbringing an objective lens close to the optical disc while irradiatingthe optical disc with leaser beam, an FE signal measuring step formeasuring an amplitude of a focus error signal during said driving stepand for storing a maximum value FEmax and a minimum value FEmin of saidfocus error signal, and a discriminating step for comparing a differencebetween an absolute value of the maximum value FEmax and an absolutevalue of the minimum value FEmin which are stored in said FE signalmeasuring step with a previously stored discrimination value, whereinthe kind of the optical disc is discriminated in said discriminatingstep.
 7. The optical disc discriminating method according to claim 6,wherein in said discriminating step, (FEmax+FEmin)/(FEmax−FEmin) iscalculated, said calculated (FEmax+FEmin)/(FEmax−FEmin) is compared withthe previously stored discrimination value, thereby discriminating thekind of the optical disc.
 8. The optical disc discriminating methodaccording to claim 6 or claim 7, wherein a value obtained by calculatingthe maximum value FEmax and the minimum value FEmin based on the focuserror signal at a position where said leaser beam comes into focus isused as said discrimination value.
 9. The optical disc drive accordingto any one of claims 1 to 5, wherein the optical disc drive uses apickup in which a focus deviation is generated between leaser beam whichpasses through an outer periphery of said objective lens and leaser beamwhich passes through an inner periphery of said objective lens by adifference in base material thickness of optical disc.